If you install Manjaro on bare metal you need to change some firmware settings also known as BIOS settings.
Most systems is based on UEFI and comes with Windows as the default OS. As a security measure - such systems defaults to prevent boot of other systems.
You need to consult your system manual or device manufacturers support to obtain knowledge on how to change the following firmware settings.
To be able to boot a Manjaro live ISO you need to access the systems firmware and disable Secure Boot.
Many systems provides a shortcut to override the default boot media such as pressing F7 or F12 during system start.